Conventionally, since flake-shaped metal pigments (which will be hereinafter also referred to as “flaky metal pigments”) exhibit an excellent metallic texture when being used to form a coating film, such pigments have been used for paint, ink, and the like. Such flaky metal pigments have been conventionally manufactured by the ball mill method as described below.
First, metal powder as a raw material, an organic solvent, and a grinding aid such as a higher fatty acid are first prepared. Then, these materials are introduced into a cylindrical drum, into which media (balls) are introduced. Then, the drum is rotated to apply mechanical force to the metal powder within the drum, thereby allowing flaking of the metal powder.
Such a ball mill method is suitable to manufacture powder having an average particle diameter of 10 μm or more and the maximum particle diameter of more than 20 μm. Thus, the manufactured powder is widely utilized in coating techniques such as spray coating and screen printing.
In recent years, a coating technique implemented by ink jetting has been started to be employed in place of the coating technique as described above. Accordingly, there has been an increased demand for metallic printing by which a metallic image and the like are printed using this coating technique. However, when flaky metal pigments produced by the conventional ball mill method are used in ink jetting for the purpose of conducting metallic printing, the following problem occurs.
In ink jetting, ink is discharged at high speed from an ink-jet nozzle and the discharged ink is coated on a base body such as a paper medium to form a coating film. Thus, an image is formed by arrangement of this coating film. However, a flaky metal pigment is not small enough for the diameter of the hole of the ink-jet nozzle used in an industrially-applied or non-commercially-applied general ink jetting process, which causes blockage in the ink-jet nozzle, with the result that ink cannot be discharged.
In order to solve the above-described problems, an attempt has been made to produce flaky metal pigments reduced in particle diameter, thereby allowing metallic printing implemented by ink jetting. For example, Japanese Patent Laying-Open No. 2008-174712 (Cited Document 1) discloses a method of forming an aluminum vapor deposition layer by the vacuum vapor deposition method, which is subjected to an ultrasonic treatment in a solvent so as to be peeled, fine-grained and distributed, thereby producing an aluminum pigment having a particle size smaller than that in the conventional case.